Fuel cell stack having foil interconnects and laminated spacers

ABSTRACT

Interconnects and perimeter spacers for a fuel cell stack are provided as flexible elements which can conform to non-planarities in a stack&#39;s electrolyte elements and thereby avoid inducing torsional stresses in the electrolyte elements. The interconnects are foil elements about 0.005 inches thick, formed of a superalloy such as Hastelloy, Haynes 230, or a stainless steel. The perimeter spacers comprise a plurality of laminate thin spacer elements, each thin spacer element being a laminate of superalloy and a “soft” material such as copper, nickel, or mica. The spacer elements can slide past one another; thus the perimeter spacers can be physically thick, to form the gas flow spaces within the stack, while also being torsionally flexible.

TECHNICAL FIELD

The present invention relates to fuel cells; more particularly, tostacks comprising a plurality of individual cells being both physicallyseparated and electrically connected by interconnect elements; and mostparticularly, to such a fuel cell stack wherein the interconnectelements are thin foils and the spacers are laminates of foils formedalternately of superalloy and a compliant material.

BACKGROUND OF THE INVENTION

Fuel cells which generate electric current by controllably combiningelemental hydrogen and oxygen are well known. In one form of such a fuelcell, an anodic layer and a cathodic layer are separated by a permeableelectrolyte formed of a ceramic solid oxide. Such a fuel cell is knownin the art as a “solid oxide fuel cell” (SOFC). Either pure hydrogen orreformate is flowed along the outer surface of the anode and diffusesinto the anode. Oxygen, typically from air, is flowed along the outersurface of the cathode and diffuses into the cathode. Each O₂ moleculeis split and reduced to two O⁻² ions at the cathode/electrolyteinterface. The oxygen ions diffuse through the electrolyte and combineat the anode/electrolyte interface with four hydrogen ions to form twomolecules of water. The anode and the cathode are connected externallythrough the load to complete the circuit whereby four electrons aretransferred from the anode to the cathode. When hydrogen is derived from“reformed” hydrocarbons, the “reformate” gas includes CO which isconverted to CO₂ at the anode/electrolyte interface. Reformed gasolineis a commonly used fuel in automotive fuel cell applications.

A single cell is capable of generating a relatively small voltage andwattage, typically about 0.7 volts and less than about 2 watts per cm²of active area. Therefore, in practice it is usual to stack together inelectrical series a plurality of cells. Because each anode and cathodemust have a free space for passage of gas over its surface, the cellsare separated by perimeter spacers which are vented to permit flow ofgas to the anodes and cathodes as desired but which form seals on theiraxial surfaces to prevent gas leakage from the sides of the stack.Adjacent cells are connected electrically by “interconnect” elements inthe stack, the outer surfaces of the anodes and cathodes beingelectrically connected to their respective interconnects by electricalcontacts disposed within the gas-flow space, typically by a metallicfoam or a metallic mesh which is readily gas-permeable or by conductivefilaments. The outermost, or end, interconnects of the stack defineelectrical terminals, or “current collectors,” connected across a load.

In the prior art, the interconnect elements are relatively thick, flatplates formed of a superalloy or stainless steel. Also, the perimeterspacers that form the gas flow spaces adjacent to the electrodes aretypically formed from sheet stock having a thickness selected to yield adesired height of the flow space.

One problem encountered in prior art fuel cell stacks is that they arerelatively bulky and heavy. It is very desirable to reduce the heightand weight of a stack without sacrificing performance.

Another problem encountered in some prior art fuel cell stacks involvesthe brittleness of the ceramic oxide electrolyte elements. In some fuelcells, the anode is a relatively thick structural element supporting athin electrolyte layer and a thin cathode layer. Such a fuel cell issaid to be “anode-supported.” The ceramic oxide electrolyte elements,which extend to the edges of the stack in contact with the anodes,typically are not optically flat and are also quite brittle. The anodesmay also not be optically flat. Prior art perimeter spacers, beingmonolithic, cannot twist to accommodate non-planarities in theelectrolyte elements and anodes, so that sealing between the non-flatsurfaces becomes difficult. Also, because of the non-flat surfaces, anelectrolyte element may be cracked during assembly of the stack. Ineither case, failure of the stack can occur. Avoiding these problems byfinishing the electrolyte elements to be optically flat iscost-prohibitive.

It is a principal object of the present invention to provide a fuel cellstack that is lighter and smaller than prior art fuel cells of the sameelectrical capacity.

It is a further object of the present invention to provide spacer meansfor a fuel cell stack that can sealably conform to non-planarities inthe electrolyte elements and will not induce torsional stress in suchelements.

SUMMARY OF THE INVENTION

Briefly described, the interconnects and perimeter spacers for a fuelcell stack are provided as flexible elements which can conform tonon-planarities in a stack's elements. The interconnects are foilelements about 0.005 inches thick, formed of a superalloy, such asHastelloy or Haynes 230, or stainless steel. The thick perimeter spacerscomprise a plurality of thin spacer elements. Each spacer element is alaminate of a superalloy and a compliant soft material such as copper,nickel, or mica. The spacer elements can slide past one another; thusthe perimeter spacers can be physically thick, to form gas flow spaceswithin the stack, while also being torsionally flexible.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will be morefully understood and appreciated from the following description ofcertain exemplary embodiments of the invention taken together with theaccompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of a prior art two-cell stackof solid oxide fuel cells;

FIG. 2 is an isometric view of a fuel-cell stack comprising five cells;

FIG. 3 is an isometric view like that shown in FIG. 2, showing theaddition of current collectors, end plates, and bolts to form a completefuel cell stack (exploded view) ready for use; and

FIG. 4 is a schematic cross-sectional view of a two-cell stack of solidoxide fuel cells in accordance with the invention, showing the use of alaminate foil spacers and a foil interconnect element for providing thereformate and air flow passageways across the anodes and cathodes,respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a prior art fuel cell stack 10 includes elementsnormal in the art to solid oxide fuel cell stacks comprising more thanone fuel cell. The example shown includes two fuel cells A and B,connected in series, and is of a class of such fuel cells said to be“anode-supported” in that the anode is a structural element having theelectrolyte and cathode deposited upon it. Element thicknesses as shownare not to scale.

Each fuel cell includes an electrolyte element 14 separating an anodicelement 16 and a cathodic element 18. Each anode and cathode is indirect chemical contact with its respective surface of the electrolyte,and each anode and cathode has a respective free surface 20,22 formingone wall of a respective passageway 24,26 for flow of gas across thesurface. Anode 16 of fuel cell B faces and is electrically connected toan interconnect 28 by filaments 30 extending across but not blockingpassageway 24. Similarly, cathode 18 of fuel cell A faces and iselectrically connected to interconnect 28 by filaments 30 extendingacross but not blocking passageway 26. Similarly, cathode 18 of fuelcell B faces and is electrically connected to a cathodic currentcollector 32 by filaments 30 extending across but not blockingpassageway 26, and anode 16 of fuel cell A faces and is electricallyconnected to an anodic current collector 34 by filaments 30 extendingacross but not blocking passageway 24. Current collectors 32,34 may beconnected across a load 35 in order that the fuel cell stack 10 performselectrical work. Passageways 24 are formed by anode spacers 36 betweenthe perimeter of anode 16 and either interconnect 28 or anodic currentcollector 34. Passageways 26 are formed by cathode spacers 38 betweenthe perimeter of electrolyte 14 and either interconnect 28 or cathodiccurrent collector 32.

Referring to FIGS. 2 and 3, a plurality of individual fuel cells may bestacked together to form a stack 12 similar to schematic stack 10 shownin FIG. 1. Stack 12 comprises five such cells. To form a completeworking fuel cell assembly 13 (FIG. 3), stack 12 is sandwiched betweenan anodic current collector 34 and a cathodic current collector 32 whichin turn are sandwiched between a top plate 15 and a gas-manifold base17, the entire assembly being sealingly bound together by bolts 19extending through bores in top plate 15 and threadedly received in boresin base 17.

Preferably, the interconnect and the current collectors are formed of analloy, typically a “superalloy,” which is chemically and dimensionallystable at the elevated temperatures necessary for fuel cell operation,generally about 750° C. or higher, for example, Hastelloy, Haynes 230,or a stainless steel. The electrolyte is formed of a ceramic oxide andpreferably includes zirconia stabilized with yttrium oxide (yttria),known in the art as YSZ. The cathode is formed of, for example, porouslanthanum strontium manganate or lanthanum strontium iron, and the anodeis formed, for example, of a mixture of nickel and YSZ.

In operation (FIG. 1), reformate gas 21 is provided to passageways 24 ata first edge 25 of the anode free surface 20, flows parallel to thesurface of the anode across the anode in a first direction, and isremoved at a second and opposite edge 29 of anode surface 20. Hydrogenand CO diffuse into the anode to the interface with the electrolyte.Oxygen 31, typically in air, is provided to passageways 26 at a firstedge 39 of the cathode free surface 22, flows parallel to the surface ofthe cathode in a second direction orthogonal to the first direction ofthe reformate (second direction omitted for clarity in FIG. 1), and isremoved at a second and opposite edge 43 of cathode surface 22.Molecular oxygen gas (O₂) diffuses into the cathode and is catalyticallyreduced to two O⁻² ions by accepting four electrons from the cathode andthe cathodic current collector 32 (cell B) or the interconnect 28 (cellA) via filaments 30. The electrolyte is permeable to the O⁻² ions whichpass by electric field through the electrolyte and combine with fourhydrogen atoms to form two water molecules, giving up four electrons tothe anode and the anodic current collector 34 (cell A) or theinterconnect 28 (cell B) via filaments 30. Thus cells A and B areconnected in series electrically between the two current collectors, andthe total voltage and wattage between the current collectors is the sumof the voltage and wattage of the individual cells in a fuel cell stack.

FIG. 4 shows an improved two-cell stack 44 in accordance with theinvention. Stack 44 is similar to prior art stack 10 shown in FIG. 1 butincorporates several improvements over the prior art.

First, prior art interconnect element 28 is relatively thick, heavy, andinflexible, having a typical thickness of about 3.5 mm or greater. Itsthickness and weight contribute to the overall weight and size of aprior art stack, and its inflexibility contributes to the risk ofcracking a non-planar electrolyte element or compromising thesealability between elements, as discussed above. Improved interconnectelement 28 a is formed as a flexible foil having a thickness of lessthan about 0.5 mm and preferably about 0.127 mm (0.005 inch). Like theprior art interconnect 28, foil interconnect 28 a is preferably formedof an alloy which is chemically and dimensionally stable at the elevatedtemperatures necessary for fuel cell operation, for example, asuperalloy such as Hastalloy, Haines 230, or a stainless steel.

Second, prior art anode spacer 36 and cathode spacer 38 are monolithicand formed from sheet stock having a thickness selected to yield thedesired height of the anode passageways 24 and cathode passageways 26.Being inflexible, the prior art spacers also contribute to the risk ofcracking a non-planar electrolyte element upon assembly of the stack asshown in FIG. 3, and/or compromising the sealability between theelements. As shown in FIG. 4, prior art spacers 36,38 are replaced byimproved spacers 36 a,38 a, each comprising a plurality of thin elements46, each preferably about 0.127 mm in thickness. Each element 46 isformed as a laminate comprising a superalloy and a soft and malleablematerial such as copper, nickel, or mica. Preferably, a foil ofdielectric soft material, such as mica 48, is placed next to each ofelectrolyte elements 14 and anodes 16 to insulate the foil interconnect28 a electrically from short-circuiting through the electrolyteelements, as shown in FIG. 4, and to seal against gas leakage.Phologopite or fluorophlogopite mica is especially effective at sealingagainst the irregular and porous surface of the electrolyte.

Using a plurality of thin spacers allows the height of the flow spacesto be varied as may be desired among different fuel cell stacks simplyby varying the number of spacers included for each flow space. Forexample, in a currently preferred embodiment of a fuel cell stack,cathode flow space 26 is formed by five such spacers and anode flowspace 24 is formed by three such spacers. Further, selection of spacermaterials according to their thermal expansion properties allows theyield strength and thermal expansion of a stack to be specified, andthermal expansion of the spacers can provide sealing force for the sealsagainst the electrolyte elements and the interconnect and currentcollectors. Further, forming thick spacers by assembling a plurality ofthin laminated foil spacers, such that the foils may slide past oneanother as needed, results in a thick spacer which is nonethelesssufficiently flexible to conform to a non-planar electrolyte elementwithout cracking it.

While the invention has been described by reference to various specificembodiments, it should be understood that numerous changes may be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but will have full scope defined by the languageof the following claims.

1. A perimeter spacer for use in a fuel cell stack, comprising betweentwo and ten laminate spacer elements positioned adjacent to one another,at least one of said spacer elements being formed of a superalloy and asoft material, wherein said soft material is selected from the groupconsisting of copper and dielectric materials and wherein at least oneof the outermost of the laminate spacer elements is formed of mica forsealing and insulating disposition against an electrolyte element insaid fuel cell stack.
 2. A perimeter spacer in accordance with claim 1wherein said dielectric material is mica.
 3. A perimeter spacer inaccordance with claim 1 configured for spacing an electrolyte elementfrom an interconnect element, wherein said perimeter spacer includesbetween two and ten laminate spacer elements.
 4. A perimeter spacer inaccordance with claim 2 wherein at least one of the outermost of thelaminate spacer elements is formed of mica for sealing and insulatingdisposition against an electrolyte element in said fuel cell stack.